1. Field of the Invention
The present invention relates to a continuous process for preparing aluminum hydroxide. In particular, the present invention relates to a process for continuously preparing aluminum hydroxide which contains no coarse particles by hydrolysis of its precursor at high productivity in industry.
2. Description of the Related Art
Attention is paid to a hydrolysis of a metal alkoxide as a synthesis method of a sol, gel or fine particles which are precursors of ceramics because of advantages that:
(1) the metal alkoxide is easily hydrolyzed at room temperature to provide a hydroxide, and
(2) there is no possibility that a product contains impurity anions
(see Amer. Ceram. Soc. Bull., 54, 286 (1975) and Nippon Ceramic Kyokai Gakujutsu Ronbunshi, 99 (10), 1036-1046 (1991)).
When a silicon alkoxide or titanium alkoxide is used as a raw material, monodisperse spherical particles containing no coarse particle can be easily prepared by the hydrolysis (see, for example, J. Colloid and Interface Sci., 26, 62 (1968) and J. Am. Ceramic Soc., 65, C199 (1982)). But, when an aluminum alkoxide is used as a raw material, gel-form precipitate tends to form since the hydrolysis reaction rate is high, so that it is difficult to produce monodisperse particles having uniform particle size and containing no coarse particle.
Aluminum oxide which is prepared by calcining aluminum hydroxide is widely used as a sintering raw material, a filler and the like. For the purpose of excellent properties, aluminum oxide which has a narrow particle size distribution and is easily dispersed is desired.
In the synthesis of aluminum hydroxide by hydrolysis of an aluminum alkoxide, attempts have been made to prepare monodisperse particles under certain specific conditions (see Japanese Patent KOKAI Publication No. 158116/1987 and J. Am. Ceramic Soc., 74, 2263 (1991)).
Those approaches use the hydrolysis reaction at a low concentration of the raw material in a reaction medium to which a solvent other than an alcohol constituting the aluminum alkoxide is added. While the monodisperse particles containing no coarse particle can be prepared, the productivity is low in the industrial scale production. Further, when the alcohol used as the solvent is recycled to the synthesis of aluminum alkoxide, it should be purified.